The Dangers of Cassava (Tapioca) Consumption.

Cassava (Tapioca) is a worldwide staple food consumed by over 800 million people. It contains cyanide which may lead to acute toxicity or chronically may be an aetiological factor in tropical nutritional amblyopia, tropical neuropathy, endemic goitre, cretinism and tropical diabetes. It may also have carcinogenic potential. However, despite nutritional limitations it has many advantages as a crop to the subsistence farmer and would be difficult to replace.


INTRODUCTION
Cassava also known as tapioca, manioc or yuca is a aPle food for several hundred millions of people world-, 'de (Hahn and Keyser, 1985). It has been cultivated for etween 2500 and 4000 years in the Americas and was 'scovered by the conquistadores from the Old World g ?ck, 1982). Cassava was brought to West Africa from ^razil by the Portugese slave traders of the 16th century ?nes, 1959) its cultivation increasing rapidly in Nigeria j^d throughout tropical Africa during the 19th and early th centuries (Lancaster et al, 1982). It was taken to India the 17th century and cassava was adopted as a cash ?P by the colonial authorities in Southeast Asia in the 'd-19th century, large plantations being established ? Produce starch and pearl tapioca (Brautlecht, 1953 (Lancaster and ??ks, 1983). Harvesting is carried out between 9 and 24 months depending on the variety (Purseglove, 1974).
Cassava will grow between latitudes 30? N and 30? S of the equator, up to 2000 m above sea level, in temperatures from 18? to 20? C and in rainfall of 50 to 5000 mm annually (Okigbo, 1980).

CASSAVA PRODUCTION WORLDWIDE
World cassava production for 1983 was estimated at 123 million metric tons (Table 1). During the last 20 years, total cassava production has increased at the same rate as population growth in developing countries. This increase is largely due to increases in area planted since yields have remained constant around 9 tons per hectare (far below the maximum experimental yield of 80 tons per hectare (International Centre for Tropical Agriculture, 1979)).
Approximately 65 per cent of the total cassava production is used for direct human consumption and of this, about half is eaten after the fresh roots are cooked and the other half is processed in a number of different ways to make flours or meals. At present approximately 500 million people consume 300kcals per day as cassava, in Africa 50 million people consume 500kcal/day and in southern India 25 million people consume over 700kcal/ day (Cock, 1982).
Freshly harvested cassava tubers are more highy perishable than other major root crops but the subsistence farmer's way of overcoming this is to leave the crop in the ground until needed (Richard and Coursey, 1981).

NUTRITIONAL LIMITATIONS
The nutrient composition is shown in Table 2. The fresh cassava root is mainly water and carbohydrate although it is relatively rich in vitamin C and calcium. It is poor in protein and other vitamins and minerals. The amino acid profile is low in some essential amino acids, particularly the sulphur containing methionine (Okigbo, 1980), and the protein content is reduced still further by the traditional processing methods (Lancaster et al, 1982). Vitamin C content is reduced considerably during processing and in some types of cassava flour no vitamin C remains.
Much of the thiamin and niacin found in the raw root is also lost particularly during the washing processes.
Many nutritionists regard cassava, with its almost exclusively carbohydrate content, as unsuitable as a main  (University of Georgia, 1972) but this is to deny its value as a major source of energy, up to 10% world wide (Anonymous, 1973) and between 25 and 55% in some parts of southern Nigeria (Nicol, 1952). Clearly in times of drought during which cassava may be the only crop to survive, or other food shortage circumstances, reliance solely on cassava will produce nutrient inbalance and deficiency, but alternatives in such situations do not readily present themselves.
The nutritional contribution of cassava leaves which are a good source of protein and vitamins (Table 2) is frequently ignored in many countries that consume the tubers (Hall, 1986). Unfortunately the leaves are often regarded as poor man's food and only eaten under stress conditions as during the Nigerian Civil War (Anonymous, 1969). Recent analyses have reported protein contents of between 6.3 and 11.8 g/1 OOg fresh weight (Lancaster and Brooks, 1983) which compare favourably with rice and maize. Although the amino acid values of cassava leaf protein exceed those in the FAO reference protein for most essential amino acids, there is a deficiency of the sulphur containing methionine. If the relatively cheap synthetic methionine is added the net protein utilization increases from 32 to 61 per cent (Luyken et al, 1961).

TOXIC EFFECTS OF CASSAVA CYANOGENETIC GLYCOSIDES
For centuries it has been recognised that cassava may have an acute toxic effect (Clusius, 1605) and that this is related to the content of cyanogenetic glycosides was first demonstrated in 1836 (Henry and Boutron, 1836). Cassava contains 2 cyanogenetic glycosides, linamarin (which accounts for 90% of the total) and lotaustralin.
These hydrolyse in the presence of endogenous = linamarinase, released by bruising, soaking etc to liberate hydrogen cyanide (HCN) (Montgomery, 1980). Traditionally cassava is classified as bitter or sweet depending on the cyanogen content, the former having a high level distributed throughout the tuber, but whether there is a close correlation with taste has not been established (Lancaster et al, 1982). Although sweet varieties contain cyanogen it is concentrated mainly in the outer layers, the phelloderm. The cyanide content of the plants is increased in drought (Ministry of Health, Mozambique, 1984), and potassium deficiency (Oke, 1968).
The normal range of cyanogen content of cassava tubers falls between 15 and 400 mg HCN/kg fresh weight (Lancaster et al, 1982) and the traditional methods of processing?drying, boiling, soaking and fermentation? are variably effective at reducing the cyanogen content.
Inorganic cyanide is rapidly absorbed from the gastrointestinal tract and reacts with thiosulphate (for which sulphur containing amino acids are essential (Cliff et al., 1985)) to form thiocyanate. Vitamin B12, as hydroxocobalamin is also essential in the detoxification of cyanide (Osuntokum, 1981).

ACUTE TOXICITY
The minimum lethal dose of HCN for humans taken orally lies between 0.5 mg and 3.5 mg per kg of body weight (Montgomery, 1980), and deaths after heavy meals of cassava continue to be reported in the Nigerian press (Osuntokun, 1981). Non-fatal acute cases of toxicity have taken the form of drowsiness, weakness and vomiting in children (Cheok, 1978) or irreversible spastic paraparesis (Ministry of Health, Mozambique, 1984). This latter occurrence involved over 1000 persons in Mozambique and was associated with a severe drought which increased the cyanide content of the cassava, the only crop to survive in any quantity. Because of the lack ? other foodstuffs there was an absence of sulphur con taining amino acids in the diet, and the normal length traditional processing of the cassava was abbreviated-CHRONIC TOXICITY ^ Chronic intoxication is more common in Nigeria and ere is now much circumstantial evidence to incriminate the cyarnde content of cassava as an aetiological factor in n^th r?Plca' nutritional amblyopia and tropical neuro pathy (Osuntokun, 1981;Ayanru, 1976). In two recent reports, all patients gave a history of longstanding regu' iar cassava consumption, were of low socio-econormc atus and several had signs of vitamin B deficiencies-Av^n Wm6-, yitamin B12 deficient (Osuntokun, 1981(Osuntokun, ' ayanru, 1976.
Endenrc goitre and cretinism have been linked to tir>nC^n?at0 Proc^uct'on secondary to cassava consumP n.
, 'ocyanate blocks iodine uptake by the thyr?|d /r?, H3n exacer':>ates any pre-existing iodine deficiency r 0ur a ': 1978). Endemic goitre is prevalent in i ? ^'9er'a 'n areas of iodine deficiency and 1 /j/-11 ^ at cassava consumption is a contributory facto (Kelly and Snedden, 1960 (Oyefeso, 1976). Some of trJg newer methods, however, such as screw processing a significantly quicker and more effective (Oben and 1981). Other strategies to minimise toxicity rely ?n j awareness of 'at risk' groups. Education during times bought that full processing procedures should be followed, may be possible. Vitamin B12 deficiency should guarded against and treated if found, and programmes to improve protein energy malnutrition, with partlcular attention to sulphur containing amino acids might instituted. In areas of iodine deficiency, administratis of iodised oil has been effective in the prevention of 9?itre (Cock, 1982). ^

ADVANTAGES OF CASSAVA
Respite the dangers and drawbacks both from a nutrient and toxicological standpoint, cassava is one of the most lrnPortant foodstuffs of the tropical world, and has been Ascribed as the 'tropical staff of life' (Rickard and Coursey, 1981). As Hahn and Keyser comment (1985), How a crop that contributes significantly to the diets of ?ver 800 million people (worldwide) can be virtually Upiknown beyond its area of consumption is part of the nature of subsistence farming". Cassava is a particularly Va'uable crop to the subsistence farmer giving a higher energy productivity than other staples (Coursey and ^aVnes, 1970). It is not season bound, has a low cost of deduction with low labour requirements and easy cultlvation, has the ability to grow in suboptimal soils and ac*s as a famine security crop available for harvest as needed (University of Georgia, 1972). It is resistant to 'nsect pests particularly the migrating African locust 'Wood, 1965).
Cassava also has many advantages as a livestock feed, ar,d numerous non-food uses. There is much scope for lriCreasing the export of cassava products. SHOULD CASSAVA BE REPLACED? advantages of the crop to the subsistence farmer, T16 extent of its cultivation and the significant contribu-?n it makes to the energy intake of much of the world's ^Pulation militate against an easy replacement by fn?ther staple, even if an appropriate crop could be Und. Cultural resistance would be strong and one author at least believes that there would need to be Severe compulsion to change to a more labour intensive Cr?P such as rice (Hart, 1982). FUTURE RESEARCH n past the distinction between bitter and sweet ^ r,eties was felt to be blurred with certain varieties e'ng inocuous if grown in one area and poisonous if r r?Wn in another (Lancaster et al, 1982). More recently, aln?arch ^as demonstrated consistently sweet varieties ?ugh these are generally limited by low yields corned to bitter varieties. Breeding experiments have sugsted that a reliable high yielding sweet variety will be r?duced in the near future (Oben and Menz, 1981 Such developments in reducing the toxicity and im-Vln9 the nutritional contribution of cassava would go a long way towards removing the dangers currently associated with cassava consumption.